Protected resonator

ABSTRACT

A bulk acoustic wave resonator structure that isolates the core resonator from both environmental effects and aging effects. The structure has a piezoelectric layer at least partially disposed between two electrodes. The structure is protected against contamination, package leaks, and changes to the piezoelectric material due to external effects while still providing inertial resistance. The structure has one or more protective elements that limit aging effects to at or below a specified threshold. The resonator behavior is stabilized across the entire bandwidth of the resonance, not just at the series resonance. Examples of protective elements include a collar of material around the core resonator so that perimeter and edge-related environmental and aging phenomena are kept away from the core resonator, a Bragg reflector formed above or below the piezoelectric layer and a cap formed over the piezoelectric layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/321,860, filed on Jan. 26, 2009, which is scheduled to issue as U.S.Pat. No. 8,030,823 on Oct. 4, 2011, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

High-stability resonators and, more particularly, thin-film bulkacoustic wave resonators that are largely immune to environmentaleffects and aging are disclosed herein.

BACKGROUND OF THE INVENTION

The response of a bulk-acoustic wave resonator (FBARs, SMRs, HBARs,etc.) exhibits long term drift in its characteristics, particularly infrequency. This time-dependent long-term change is known as drift of theresonator. The drift is caused by both intrinsic and extrinsic factorsand the intrinsic instability is often called as aging of the resonator.Aging occurs even when external environmental factors are kept constant.An example of an SMR structure from the prior art is illustrated in FIG.1.

In the literature, Walls and Vig, “Fundamental Limits on the FrequencyStabilities of Crystal Oscillators,” IEEE Transactions On Ultrasonics,Ferroelectrics, And Frequency Control, 42(4):576-589, July 1995(referred to herein as Walls and Vig, 1995) and Vig and Meeker, “TheAging of Bulk Acoustic Wave Resonators, Filters, and Oscillators,” Proc.45^(th) Ann. Symp. Frequency Control, IEEE Cat. No. 91 CH2965-2, pp.77-101, (1991) (referred to herein as Vig and Meeker 1991) present ataxonomy of mechanisms that cause aging in resonators and oscillators.These mechanisms include mass transfer to or from the resonator'ssurfaces due to deposition or removal of contaminants, stress relief inthe mounting structure of the crystal, changes in the electrodes, leaksin the package, and changes in the piezoelectric material. Othermechanisms include external environmental effects like temperature andstress cycling (hysteresis) and inertial effects.

In general, previous attempts in making a resonator stable againstenvironmental effects and aging have focused on frequency stability.This effort has focused on packaging and mounting structure design.

Usually, packaging of the resonator has been the primary method ofprotecting it against aging that is caused by contamination and leaks.Also, packaging partially insulates the resonator from externalenvironmental effects.

Quartz resonators have traditionally been packaged within containers toprotect them from certain aging phenomena. Many examples exist in theprior art. For example, see U.S. Pat. No. 5,640,746 entitled “Method ofHermetically Encapsulating a Crystal Oscillator Using a ThermoplasticShell” to Knecht et al. that issued on Jun. 24, 1997.

Micromachined thin film resonators are packaged using wafer-scale ordevice-scale encapsulation techniques. Many examples exist in the priorart. Micromachined thin film resonators like silicon resonators andthin-film bulk acoustic wave resonators (FBARs) use micromechanicalsupport structures such as posts and suspensions. These structures arealso designed to minimize the transfer of stress, includingtemperature-induced stress to the crystal resonator. For example, seeKim et al. “Frequency stability of wafer-scale film encapsulatedsilicon-based MEMS resonators,” Sensors and Actuators A 136 (2007)125-131. Also see U.S. Pat. No. 7,153,717 entitled “Encapsulation ofMEMS Devices Using Pillar-Supported Caps” to Carley et al. that issuedon Dec. 26, 2006.

The current methods of packaging are either high profile (the case withquartz), which makes them difficult to integrate in a product; or theyencapsulate thin-film structures like released inertial resonators orFBARs that are susceptible to other forms of instability such asacceleration or shock.

The mounting structure is another location for possible aging. Stressintroduced by packaging and transmitted to the crystal causes long-termfrequency aging.

Quartz resonators have been mounted via support legs before being sealedunder a cap. The support structure is carefully designed to minimizestress transfer (hence aging) to the crystal. Many examples exist in theprior art. For example, see U.S. Pat. No. 4,642,510 entitled “Mount forquartz crystal oscillator device” to Yamashita that issued on Feb. 10,1987. See also U.S. Pat. No. 5,030,875 entitled “Sacrificial QuartzCrystal Mount” to Knecht, that issued on Jul. 9, 1991. Levitating thecrystal using electrostatic levitation so that aging effects related toa mechanical mounting structure are minimized has also been suggested.See Wall and Vig, 1995.

The current methods of mounting of the resonator are susceptible toinertial and thermal fatigue, hence aging. None of these approaches,however, address the protection of the crystal and/or electrode materialitself. The current approaches do not protect the crystal or electrodematerial from environmental effects, including aging.

The research focus to date on frequency stability is appropriate forcertain applications. However, a more general focus on the entirebehavior of the resonator around the primary resonance, in bothfrequency (f) and over time is desired.

SUMMARY OF THE INVENTION

The present invention is directed to a bulk acoustic wave resonatorstructure that isolates the core resonator from both environmentaleffects and aging effects. The structure protects against contamination,package leaks, and changes to the piezoelectric material due to externaleffects like ionizing radiation and package stress, while stillproviding excellent inertial resistance. In preferred embodiments of thepresent invention the structure has one or more protective elements thatlimit aging effects to at or below a specified threshold. That thresholdis expressed herein as a shift in the impedance response of theresonator as a function of frequency that is at or below a certainvalue. As one skilled in the art will appreciate, an acoustic resonatordevice has an impedance value for each frequency value. As the resonatorages, the impedance value associated with a frequency value may changeover time. Thus, the impedance value (Z) associated with a frequency ffor the acoustic resonator as designed may change to the value Z_(i)over time. The present invention limits the rate of the change offrequency to at or below a certain level.

More specifically, an acoustic resonator, when configured in anoscillator, oscillates at a frequency f_(osc) that is associated with acomplex-valued impedance Z=(Za, ZΦ). This complex-value impedance isinitially determined when a value of a function g (Z) equals a specifieddesign value g₀. Drift in the resonator behavior in its bandwidth occurswhen the complex-valued impedance changes from the value associated withg₀. This drift manifests itself as shift in the oscillator frequency(f_(osc)) associated with the complex-valued impedance from the(f_(osc)) associated with the complex-valued impedance at g₀. The driftin f_(osc) is expressed herein as a ppm change in frequency associatedwith a specific complex-value impedance per unit time. As used herein“ppm” is Hz on a MHz scale. That is, a change of 5 Hz is a 5 ppm changein an f_(osc) of 1 MHz. The rate of change in f_(osc) is at or belowabout 5 ppm/year.

In other embodiments, this drift can be measured as a change series orparallel resonance of the resonator. One skilled in the art willappreciate that, over the resonator bandwidth, the impedance responseexhibits a minimum in amplitude that is associated with a particularfrequency. The frequency associated with this minimum in amplitude istermed the series resonance. The frequency associated with the maximumin amplitude is the parallel resonance. The effects of aging alsomanifest themselves as a change in the frequencies associated with theseries and parallel resonance. In these preferred embodiments, the shiftin at least one of the series or parallel resonance frequency of thedevice is at or below about 5 ppm per year.

According to the embodiments described herein, mitigation of the effectsthat cause aging is more than simply minimizing frequency drift of theseries or parallel resonance. For example, series resonance frequencydepends on the acoustic path and any parasitics. Therefore theelectromechanical coupling coefficient and dielectric constant of theresonator materials (and changes thereto over time) play no role indetermining the series resonance frequency, and any subsequent drift inthat frequency.

Consequently, in certain embodiments the resonator behavior isstabilized across the entire resonator bandwidth, not just at the seriesresonance. In order to achieve such stabilization, the electromechanicalcoupling coefficient and the dielectric constant of resonator materialsmust be stable over time. Embodiments of the present invention addressshort-term environmentally-driven instabilities as well as long-termaging effects on frequency drift. However, in certain preferredembodiments, the resonator behavior is stabilized such that the shift inthe resonance frequency for at least one the series resonance and theparallel resonance is less than 5 ppm per year of device operation.

The resonator is protected from the effects of degradation of thematerial due to environmental and/or aging processes, by providingprotective element that is a collar of material, in one particularembodiment a piezoelectric material, around the core resonator so thatperimeter and edge-related environmental and aging phenomena are keptaway from the core resonator.

In certain embodiments the resonator is protected against the effect ofcontamination on the surface by surrounding it on all sides by aprotective element that is a plurality of energy-confining Bragg layers.

In certain embodiments the package and environmental stresses areattenuated and thereby prevented from reaching the main resonatorstructure. Attenuation is accomplished by providing a protective elementthat is one or more layers formed over and around the resonatorfunction. These layers are Bragg layers, spacer layers, stress relieflayers or sealing layers, or some combination thereof, as described morefully herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of a thin-film solidly mounted prior artSMR (solidly mounted resonator);

FIG. 2 is a frequency spectrum of an electrical impedance of aresonator;

FIG. 3 is a cross section view of an alternative embodiment of thepresent invention with the core resonator surrounded by the collar;

FIG. 4 is a cross section view of alternative embodiment with the coreresonator surrounded by the collar;

FIG. 5 is a frequency spectrum view of a prior art SMR (FIG. 1);

FIG. 6 is a frequency spectrum view of an alternative embodiment;

FIG. 7 is a frequency spectrum view of a prior art SMR (FIG. 1) with thesame material as the SMR in FIG. 5;

FIG. 8 is a frequency spectrum view of an alternative embodiment withthe same materials from the SMR of FIG. 5;

FIG. 9 is a cross section view of an alternative embodiment with thecore resonator surrounded by the collar;

FIG. 10 is a cross section view of another alternate embodiment in aFBAR configuration with a core resonator, a collar and a spacer, under acap;

FIG. 11 is a cross section view of an alternate embodiment in a FBARconfiguration with a core resonator, a collar and spacer, under a cap,where the FBAR is supported at the center of the structure;

FIG. 12 is a cross section view of an alternate embodiment with aninverted SMR configuration having a core resonator and a collar andencapsulated under a cap that is constructed as a Bragg reflector;

FIG. 13 is a cross section view of an alternate embodiment with aninverted SMR configuration having a core resonator and a collar andencapsulated under a cap that is constructed as a Bragg reflector withan alternative electrode configuration; and

FIG. 14 illustrates the complex impedance response of a resonator as afunction of f_(osc).

For purposes of clarity and brevity, like elements and components willbear the same designations and numbering throughout the Figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross section view of a thin-film solidly mounted prior artSMR (solidly mounted resonator) 10. The SMR has a lower Bragg reflector12, a lower electrode 14, a piezoelectric 16 layer, and an upperelectrode 18. The lower Bragg reflector 12 has a plurality of pairs ofhigh acoustic impedance layer 24 and low acoustic impedance layers 22.Each layer (22, 24) has a thickness approximately equal to an oddmultiple of the quarter acoustic wavelength of the operational mode inthe material. The operational mode is one of several resonant modes thatare supported by the structure, and include the thickness-extensionalmode, the thickness-shear mode, etc. For each of these modes, a resonantspectrum such as that shown in FIG. 2 can be realized. Selection of anodd multiple of the quarter acoustic wavelength thickness based on aparticular mode results in maximum reflection of the perfect reflectionof the acoustic energy in that mode. It is also possible to optimize thethickness to values different from the quarter wavelength thickness toobtain good reflection of acoustic energy in two or more differentmodes.

Embodiments of the present invention reduce the drift of the impedanceof the entire resonator around the primary resonance. The primaryresonance, referring to FIG. 2, is the region of the spectrum from justbefore the series resonance 28 to just after the peak of the parallelresonance 30 of the device. The series resonance is typically defined asthe frequency at which the impedance is a minimum. The parallelresonance is often defined as the frequency at which the impedance is ata maximum. The frequency range between the two is known as the bandwidthof the resonator.

Embodiments of the present invention provide a protective element thatmitigates the drift in at least one of and preferably both, the seriesresonance and the parallel resonance over time. Preferred embodiments ofthe present invention have at least one of three features, collectivelyreferred to as protective elements, which address several of the mostimportant environmental effects and aging mechanisms that are the rootcause of these undesirable shifts in series resonance and/or parallelresonance. One such embodiment is illustrated in FIG. 3. The firstexemplary protective element is a collar 44 formed around the coreresonator 42. The collar 44 as illustrated in FIG. 3 is a regionsurrounding the core resonator and includes layers of material that formthe core region. For example, peripheral portions of the piezoelectriclayer 16 extend into and are part of the collar 44. The collar 44ensures that perimeter and edge-related environmental and agingphenomena are kept away from the core resonator 42.

The core resonator 42 is immune to the deposition of contaminantsbecause it is surrounded on all sides by Bragg layers (22, 24). SuchBragg layers are described in previously cited U.S. patent applicationSer. No. 12/002,524.

In an alternative embodiment (FIG. 4), the entire structure 52 isencapsulated in a protective element that is a low acoustic-impedance,low-density encapsulant 50 material such as aerogel. In this embodiment,the package and external stresses are not transmitted to the structure52.

Referring again to FIG. 3, the cross section of 52 has a core resonator42 surrounded by the collar 44. The structure 52 has an energy-confiningBragg reflector 48 both below (12) and above (40) the piezoelectriclayer 16. The structure also has a lower electrode 14, a spacer 38layer, an optional temperature compensating layer 32, an upper electrode18, an optional interconnect 34 layer, and a passivation 36 layer. Eachreflector has a plurality of pairs of high acoustic impedance layers 24and low acoustic impedance layers 22. Each of these Bragg materiallayers (22, 24) has a thickness equal to an odd integer multiple of thequarter acoustic wavelength of the operational mode in the material. Thespacer 38 layer can have a thickness equal to an odd integer multiple ofthe quarter acoustic wavelength of the operational mode in the material.The optional interconnect 34 layer electrically connects the resonatorelectrodes (14, 18) to pads (49) or to a via 46 down to CMOS circuits(not shown) in the substrate 20. The present invention contemplates bothsingle-ended and differential embodiments. As used herein, asingle-ended embodiment is one in which each of the top and bottomelectrodes are electrically connected to a circuit. A differentialembodiment, both of the electrodes that go to circuits are located onone side of the device. The electrode on the other side is leftfloating.

The structure illustrated in FIG. 3 is formed in the followingillustrative manner. The layered structure 52 is formed on substrate 20by depositing and patterning layers of material as follows. Layers aredeposited to form a lower Bragg reflector 12 (formed from alternatinghigh 24 and low 22 acoustic impedance layers) and a lower electrode 14.The piezoelectric 16 material is deposited on the lower electrode 14.The piezoelectric 16 material is patterned by photolithography andetching. Then, the electrode 14 and, in some cases, the reflector 12 arealso patterned by photolithography followed by one or more etch steps.An insulating spacer 38 layer is then deposited and patterned, therebyremoving spacer material from over the portion of the piezoelectric 16material from the core resonator region 42 where the device will beformed. The spacer layer therefore defines the core and collar regionsin this embodiment. An upper electrode 18 is formed over the exposedregion of the piezoelectric 16 material to create the active portion ofthe device. In products where integration of resonators on a substratehaving CMOS devices is desired, ohmic contact to the lower electrode 14is provided. In the illustrated embodiment a via 46 is formed in theinsulating spacer layer 38 down to the lower electrode 14 byphotolithography and etching. A conducting interconnect 34 is thenformed to electrically connect the electrodes 14 and 18 to the CMOSdevices formed in the substrate as required. The energy-confining Braggreflector layers 22 and 24 that form the upper Bragg structure are thendeposited in sequence and, subsequently, patterned. Preferably suchpatterning will provide access to the interconnect layer 34 forsubsequent packaging. A sealing layer 36 is then formed over thestructure to further protect and isolated the active structure from theenvironment and the effects thereof. Layer 36 is referred to as apassivation layer or sealing layer herein. Examples of materialssuitable for use as a passivation or sealing layer include siliconnitride, polyimide, or benzocyclobutene (BCB).

FIG. 4 is a cross section view of a preferred embodiment of the presentinvention again having the structure 52 having the core resonator 42surrounded by the protective element collar 44. A protective elementthat is a low-acoustic impedance material 50 is placed over the entirestructure 52. This material encapsulates the device and attenuates thetransmission of external and package stress to the device. As with theembodiment illustrated in FIG. 3, this embodiment can also besingle-ended or differential. To further reduce the transmission ofpackage stresses that will change over time, an additional layer of softmaterial such as an encapsulation resin (e.g. silicone) commonly knownas “glop top” may be deposited over the resonator after it is assembledinto a package and before overmolding of the package with plastic.Encapsulation resins are well known to one skilled in the art and arenot described in detail herein.

The material formed over structure 52 is a low-density, low-acousticimpedance encapsulant 50 material such as aerogel. Layer 50 is depositedon structure 52 before plastic packaging. Layer 52 ensures that externaland package stresses are not transmitted to the core resonator structure42.

As previously noted, the collar 44 around the core resonator 42illustrated in FIGS. 3 and 4 isolates the core resonator 42 from mostenvironmental effects. The collar attenuates temperature, stress oraging effects before they reach the core resonator 42. Althoughapplicants do not wish to be held to a particular theory, applicantsbelieve that the collar 44 acts like a parasitic resonator on the coreresonator 42. Advantageously, the size of the parasitic resonance can becustomized during device design to meet product performancerequirements.

Also as previously noted, the use of energy-confining Bragg reflector 48is a protective element that isolates the core resonator 42 fromexternal contaminants released by the environment, as well as providingadditional shielding for the core resonator 42 against temperatureand/or stress effects. Protective element 50 is a stress barrier thatprotects the core resonator from external sources (e.g., the package,the environment) to prevent these stresses from propagating to the coreresonator 42.

The spacer 38 is another protective element that is also used toelectrically isolate the lower electrode 14 from the upper electrode 18in addition to providing extra material in the collar 44 region toattenuate environmental effects. In this regard it is advantageous ifthe spacer layer 38 is a dielectric material (e.g., silicon dioxide). Ina preferred embodiment the spacer 38 layer thickness is approximatelyequal to an odd integer multiple of the Bragg wavelength in thematerial. Bragg wavelength is defined by the following equation:

v=v(c33/d)  (1)

where v is the acoustic velocity in the material, c33 is the stiffnesscoefficient of the material in the direction of wave propagation, and dis the material density. The layer thickness is calculated by thefollowing equation:

t=N×(v/f)/4  (2)

where t is the layer thickness and f is the Bragg center frequency. N isan odd integer≧1.

Referring to FIG. 14, the impedance response for an oscillator isillustrated as a function of f_(osc). The impedance response isillustrated for an oscillator type resonator device. The deviceoscillates at a frequency f_(osc) that is determined by thecomplex-valued impedance Z=(Za, ZΦ). This impedance value is determinedwhen a value of a function g(Z) equals a specified design value g₀.Drift in the resonator behavior in its bandwidth results in a drift inthe value of f_(osc) where resonator impedance becomes equal to (Za,ZΦ). The drift in f_(osc) can be quantified in ppm/time. According tothe present invention, aging effects are controlled so that the changein frequency associated with a complex-valued impedance Z=(Za, ZΦ)changes at a rate of 5 ppm/per year or less.

External stress and/or ionizing radiation are examples of environmentalconditions that can change material properties of the resonator.Piezoelectric 16 properties, in particular, are susceptible to changedue to external factors. FIG. 5 illustrates the resonance spectrum of anSMR from the prior art (the SMR illustrated in FIG. 1). As noted above,ionizing radiation or other external process can change the materialstructure of the resonator layers, and hence their electromechanicalproperties. These changes manifest themselves as a change or shift inthe resonator spectrum. When there is a 10% decrease in thepiezoelectric 16 coupling coefficient in an SMR such as the one shown inFIG. 1, there is a large shift in the resonance spectrum. The FIG. 1structure has 3.5 pairs of tungsten/oxide Bragg reflector layers, an A1N(aluminum nitride) piezoelectric layer 16 and molybdenum (MO) electrodes(14,18). The series and parallel resonance was modeled over time forthis structure using a 1-D acoustical and 3-D electrical modelimplemented in Agilent Advanced Design System (ADS) 2006. For the priorart structure illustrated in FIG. 1, the effect of a 10% decrease in thecoupling coefficient on the resonance spectrum was modeled. For thisFIG. 1 structure, FIG. 5 illustrates that there is a −510 ppm (i.e. an870,570 Hz shift downward) shift in the series resonance 54, and a −2200ppm (i.e. a 3,790,600 Hz shift downward) shift in the parallel resonance56 as illustrated in FIG. 5. The greater decrease of the parallelresonance 30 compared to the series resonance 28 results in a lowered k2or bandwidth for the resonator. For many applications, such a shift inbehavior can severely and negatively impact performance.

FIG. 6 illustrates a frequency spectrum view of the structure 52 (e.g.,FIGS. 3 and 4) with the same materials from the prior art example ofFIG. 5 for the Bragg layers, piezoelectric layer and electrodes, butwith a 10% decrease in the piezoelectric 16 coupling of the collar 44material. This change, caused by the same environmental condition thatcaused the shift illustrated in FIG. 5, results in a negligible shift inthe resonance spectrum, with a +32 ppm shift in the parallel resonance56 (i.e. a 54,624 Hz shift upward), and 0 ppm shift (no shift) in theseries resonance 28. The reason for the difference observed in effect ofthe coupling coefficient on the frequency shift of the two devices isthat, in the device in FIGS. 3 and 4, the change in coupling coefficientof the piezoelectric material was largely confined to the collar region42. The piezoelectric material in the core resonator 44 had very littlechange in coupling coefficient.

In the example illustrated in FIG. 6, the shift of the resonance isbarely visible in a narrow band frequency sweep. A very narrow bandsweep reveals that there is negligible shift in the parallel resonance56 and no shift in the series resonance 28. This illustrates the abilityof the structure described herein to protect the core resonator 42 byproviding a collar 44 that mitigates adverse effects of the environmenton material properties without affecting the performance of the coreresonator 42.

FIG. 7 is a frequency spectrum view of the SMR of FIG. 1 using the samematerials as those identified in the discussion of FIG. 5. FIG. 7illustrates the shift in the resonance spectrum of an SMR due to a 0.1um thick layer of oxide contaminant on the top surface of the device(piezoelectric layer 16 in FIG. 1). The series and parallel resonancewas modeled over time for this structure using the software toolspreviously described. For the prior art structure illustrated in FIG. 1,as an example of the physical effects of aging in one scenario, a 0.1 umthick layer of oxide contaminant on the top surface of the device isexpected to form over the first year of use. Contaminants can be anymaterial, deposited at any rate over some time period. This was justused to illustrate the concept and the benefit of the upper bragg. Thiscauses a −1143 ppm shift in the series resonance (i.e. a 1,939,671 Hzshift downward) 54 and a −2461 ppm shift in the parallel resonance 56(i.e. a 4,199,697 Hz shift downward). Additionally, a spurious mode 58is introduced adjacent to the main resonance in the spectrum.

FIG. 7 illustrates the large changes that occur in the resonatorresponse as a result of the deposition of contaminants on the structure.The large negative shift in the series resonance 54 and the even largernegative shift in the parallel resonance 56 results in a lowered k2 orbandwidth for the resonator. Additionally, a spurious mode 58 isintroduced near the main resonance, further altering the behavior fromthe baseline. For many applications, such a shift in performance isunacceptable.

FIG. 8 is a frequency spectrum view for the structure 52, made from thesame materials set forth in the description of FIG. 5. The structure hasa 0.1 um thick layer of oxide contaminant on the top surface of thedevice. Contrary to the shift in spectrum caused by this layer on priorart devices, the oxide contaminant causes a negligible shift in theresonance spectrum of the device with structure 52.

Specifically, when the structure 52 is subjected to the sameenvironmental contaminant effect as the FIG. 1 prior art structure, theshift of the resonance is barely visible in a narrow band frequencysweep. As illustrated in FIG. 8, there is a +1 ppm shift in the seriesresonance 54 (i.e. a 1,697 Hz shift upward), and +1.2 ppm shift in theparallel resonance 56 (i.e. a 2,047.8 Hz shift upward). This illustratesthat the resonator is protected by the energy-confining Bragg structurethat attenuates the acoustic and electrical effect of contaminants thatdeposit on the device surface.

Introducing an aerogel-like encapsulant 50 around the device (FIG. 4) asan additional protective element further attenuates package stresses andinertial stresses by making them more remote from the resonator device.

FIG. 9 is another embodiment of the present invention. This embodimentis also configured to protect the resonator from external stresses. AnSMR structure is equipped with a protective element collar 44 aspreviously described. The resulting structure is then covered withanother protective element. That protective element is a device-levelthin-film cap 60 structure that can attenuate package stress as well asisolate the resonator from the effects of contaminants. In thisstructure, the stress management function of the structure is separatedfrom the acoustic resonator function.

The structure in FIG. 9 is fabricated in the following sequence. A lowerBragg reflector 12 (alternating high 24 and low 22 acoustic impedancelayers) is formed by depositing a plurality of alternating layers 22 and24 on substrate 20. A lower electrode 14 and piezoelectric 16 materiallayer are deposited on the Bragg reflector 12. The piezoelectric 16material is patterned by photolithography and etching. Then, theelectrode 14 and, in some embodiments, the reflector 12 are patterned byphotolithography and one or more etch steps. An insulating spacer 38layer is then deposited and patterned, to remove it from the portion ofthe piezoelectric 16 material in region 42. An upper electrode 18 isformed over the exposed region of the piezoelectric material 16 tocreate the active portion of the device. In products where integrationof resonators with CMOS devices is desired, (therefore requiring ohmiccontact to the lower electrode 14), a via 46 is formed in the insulatinglayer to the lower electrode 14 by photolithography and etching. Aconducting interconnect layer 34 is formed for electrical connection toeach of the electrodes. Up to this point, the manufacturing processfollows that described for the embodiments illustrated in FIGS. 3 and 4.

The protective element cap 60 structure is formed over theabove-described structure by first depositing a sacrificial layer overthe structure, forming the cap 60 structure thereover and thenpatterning the structure to remove the sacrificial layer. The resultingspace houses a vacuum or a noble gas in the completed device.Sacrificial layers may be made out of any material that can useunderlying exposed material as an etch stop. Silicon is one example of asuitable sacrificial material. The cap layer as illustrated is a twolayer structure; an inner protective layer 62 and an outer seal layer64. An inner protective layer 62 is deposited over the above describedsacrificial layer. The sacrificial layer has vias formed therein thatare filled with the protective layer material when the protective layeris formed on the sacrificial layer. Since these vias are filled with theprotective layer material, the protective layer material remains whenthe sacrificial layer is removed. The inner protective layer ispatterned such that it remains over the device region 42, anchored tothe substrate 20 by the protective layer material that remains after thesacrificial layer is removed. The sacrificial layers are then completelyetched away, leaving the device and the protective layer 62 asfreestanding, released structures. This is observed by the gap 63between the device structure and the protective layer 62. A seal layer64 is deposited over the structure encapsulating the device beneath thecap 60. Layer 62 can be any structurally stiff material such as alumina,silicon nitride, gold, etc. In certain embodiments, layer 62 isconfigured as a Bragg structure such as previously described. Layer 64can be a material that provides a hermetic seal to the underlyingstructure. Alumina and silicon nitride are examples of suitablematerials.

FIG. 9 is a cross section view of the alternate embodiment 66 describedabove with the core resonator 42 surrounded by the collar 44. Structure66 has a lower Bragg reflector 12, a lower electrode 14, a piezoelectric16 layer, a spacer 38 layer, an optional temperature compensating layer32, an upper electrode 18, and an optional interconnect 34 layer. TheBragg reflector 12 has alternating layers of high acoustic impedancelayer 24 and low acoustic impedance layer 22. The Bragg reflector has aplurality of pairs of these layers. Each of these material layers has athickness equal to a quarter acoustic wavelength of the operational modein the material. The cap 60 that is formed over the resonator structurehas an inner protective layer 62 and an over seal layer 64. As with theother embodiments described herein, the structure in FIG. 9 can besingle-ended or differential.

For certain applications, stresses originating from the wafer itself aresignificant enough to justify released (FBAR) structure even thoughthese structures exhibit decreased inertial resistance. FIGS. 10 and 11illustrate another alternate embodiment 66. The released FBARs have thecollar 44 and spacer 38 that protect the core resonator 42. The FBARsare covered with a cap 60 structure as described above for FIG. 9.

The structures illustrated in FIGS. 10 and 11 are formed as follows. Asacrificial release layer is deposited and patterned such that itremains under the device region 42. On the substrate 20 is formed atemperature compensating layer 32, lower electrode 14, piezoelectric 16material, and upper electrode 18. The upper electrode 18 is patterned byphotolithography and etching. The piezoelectric 16 material, the lowerelectrode 14, and the temperature compensation layer are all patterned(preferably with the same mask) using photolithography and a sequence ofetches. A second temperature compensation layer 32 is deposited andpatterned, sealing both regions 42 and 44 of the device exceptelectrical interconnects to the upper electrode 18. A conductive layer70 is then deposited and patterned that electrically connects the deviceand mechanically supports the structure after release.

The cap 60 structure is formed over the above structure. A secondsacrificial layer (not shown) is deposited and patterned. An innerprotective layer 62 is deposited and patterned such that it remains overthe device region, anchored to the substrate 20 through holes in thesecond sacrificial layer only. The sacrificial layers are etched away,leaving the device and the protective layer as freestanding, releasedstructures. A seal layer 64 is deposited, encapsulating the devicebeneath the protective membrane.

Referring to FIG. 10, the structure has a core resonator 42, a collar 44and a spacer 38, under a cap 60. The structure 66 has a lower electrode14, a piezoelectric 16 layer, a spacer 38 layer, an optional temperaturecompensating layer 32, an upper electrode 18, and an interconnect layer70. The interconnect 70 layer is also a suspension support. Thesuspension 70 supports the FBAR and is connected to the substrate 20 bythe anchor 72. The interconnect/suspension 70 is made of a metal such asaluminum or copper. In situations when the resonator is built directlyon top of a previously fabricated circuit wafer, the anchor 72 materialis also a metal, to facilitate interconnect with underlying circuitry.The cap 60 formed over the resonator structure consists of an innerprotective layer 62 and a seal layer 64.

FIG. 11 shows a variant of the FBAR embodiment illustrated in FIG. 10where the FBAR is supported by a center post 72 instead of sidesupports. The advantage of this embodiment is that temperature-relatedstresses and other environmental effects (one of the major reasons fordevice aging) do not introduce changes in the FBAR performance. This isbecause the mismatch in the temperature coefficient of the differentmaterials in the resonator structure, in particular the piezoelectricand the oxides cannot cause stress at the material interfaces.

FIG. 11 is a cross section view of this alternate center postembodiment. FIG. 11 illustrates a core resonator 42, a collar 44, andspacer 38, formed under a cap 60, where the FBAR is supported at thecenter of the structure. The structure has a lower electrode 14, apiezoelectric 16 layer, a spacer 38 layer, an optional temperaturecompensating layer 32, an upper electrode 18, and an interconnect (notshown) layer. The upper electrode 18 and the interconnect 34 layer formthe suspension 70 in the illustrated embodiment. The suspension 70supports the FBAR and is connected to the substrate 20 by the anchor 72.Substrate 20 has a barrier layer 78 formed thereon. The barrier layer isany suitable material such as, for example, silicon nitride. The cap 60is formed over the resonator structure. The cap 60 has an innerprotective layer 62 and a seal layer 64.

An alternative FBAR configuration is illustrated in FIG. 13. In thisembodiment, an interconnect 76 is disposed underneath the cap 60, andspecifically under the protective layer 62 of the cap 60. In thisembodiment the top electrode 18 is capacitively or inductively coupledto the interconnect 76 disposed underneath the cap. An alternativeconfiguration is not illustrated where the interconnect 76 is disposedon the barrier layer 78. In this configuration, the bottom electrode 14is capacitively or inductively coupled to the interconnect 76.Therefore, the bottom electrode 14 is the two part electrode in thisalternative configuration. The top electrode in the alternativeconfiguration is one part (as the bottom electrode 14 illustrated inFIG. 13). The interconnect is also in two parts in this alternateconfiguration, one part each being disposed on either side of thetemperature compensating layer disposed on the barrier layer. Theadvantage of this structure is that neither the bottom electrode 14 northe top electrode is performing a support function. Thereforedegradation in their mechanical properties due to environmental effectsor aging have no effect on resonator performance.

FIG. 12 is a cross section view of an alternate embodiment 74. Thisembodiment is an inverted SMR with a core resonator 42, collar 44, bothof which are encapsulated under a cap 40 that is constructed as a Braggreflector. The structure 74 has a lower electrode 14, a piezoelectric 16layer, and an upper electrode 18. A temperature-compensating layer isoptional and not illustrated. The cap 40 formed over the resonatorstructure consists of an inner protective layer 62 and a seal layer 64.The seal layer 64 is configured as an energy-confining upper Braggreflector. As previously described such a layer has multiple pairs oflayers, each pair having a high acoustic impedance layer 24 and lowacoustic impedance layer 22. As described above, each of these materialshas a thickness equal to a quarter acoustic wavelength of theoperational mode in the material. Individual reflector layers are notshown but illustrated as one monolithic layer 40. As with otherembodiments, this embodiment can be single-ended or differential.

This embodiment can be thought of as an inverted SMR hanging from thecap 40, with cap 40 constructed as a Bragg reflector. There is no lowerBragg reflector (e.g. 12 in the structure illustrated in FIG. 9);instead there is a cavity 80 created when a sacrificial layer is etchedaway by a release process.

For products that require extremely low-profile packaging, theaerogel-like encapsulant material in other embodiments may not besuitable. For other products that require a high degree of inertialshock resistance, a released FBAR structure may be unsuitable. The cap40 is able to protect the device 42 from package and external stresswhile still providing an extremely low-profile. Since the resonator is apart of the cap 40, it benefits from the mechanical strength of the cap40 while still being acoustically isolated from the environment by theBragg reflector characteristic of the cap 40.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

Since other modifications and changes varied to fit particular operatingrequirements and environments will be apparent to those skilled in theart, the invention is not considered limited to the example chosen forpurposes of disclosure, and covers all changes and modifications whichdo not constitute departures from the true spirit and scope of thisinvention.

1. A resonator comprising: a substrate; a piezoelectric layer formedover said substrate a least a portion of the piezoelectric layer beingdisposed between two electrodes; a temperature compensating layersurrounding the piezoelectric layer and electrodes; wherein thepiezoelectric material is supported on the substrate by an electricallyconductive material structure that electrically interconnects one ofcontact pads or semiconductor devices on the substrate with the topelectrode; a cap layer formed over, but not in contact with, thepiezoelectric material surrounded by the temperature compensating layer.2. The resonator of claim 1 wherein the cap layer is constructed as aBragg layer.
 3. The resonator of claim 1 wherein the resonator, inoperation exhibits a shift in the impedance response of the resonator asa function of frequency at a rate of about 5 ppm per year or less. 4.The resonator of claim 1 wherein the cap layer is spaced from theelectrically conductive layer, wherein the resonator further comprisesan electrically conductive layer disposed either on the substratebetween the substrate and piezoelectric layer or on the underside of thecap layer and disposed between the cap layer and the piezoelectricmaterial.
 5. The resonator of claim 4 wherein the electricallyconductive layer is formed on the underside of the cap layer and iselectrically coupled to the top electrode in a manner selected from thegroup consisting of capacitive coupling and inductive coupling.
 6. Theresonator of claim 4 wherein the electrically conductive layer is formedover the substrate and is electrically coupled to the bottom electrodein a manner selected from the group consisting of capacitive couplingand inductive coupling.
 7. A resonator comprising: a substrate; apiezoelectric layer formed over said substrate a least a portion of thepiezoelectric layer being disposed between two electrodes; a temperaturecompensating layer substantially surrounding the piezoelectric layer andelectrodes; wherein the top electrode is electrically interconnected toone of contact pads or semiconductor devices on the substrate through awindow in the temperature compensation material and wherein thepiezoelectric material is supported on the substrate by a post that isless than completely laterally coextensive with the piezoelectric layer;a cap layer formed over, but not in contact with, the piezoelectricmaterial surrounded by the temperature compensating layer.
 8. Theresonator of claim 7 wherein the resonator, in operation, exhibits ashift in the impedance response of the resonator as a function offrequency at a rate of about 5 ppm per year or less.
 9. The resonator ofclaim 8 wherein the cap layer is constructed as a Bragg layer.
 10. Aresonator comprising: a substrate; a piezoelectric layer formed oversaid substrate a least a portion of the piezoelectric layer beingdisposed between two electrodes; a protective element formed over thepiezoelectric layer and its associated electrodes wherein the resonator,in operation, has both a parallel resonance and series resonance andwherein the protective element is configured such that the resonator, inoperation, exhibits a shift in the impedance response of the resonatoras a function of frequency at a rate of about 5 ppm per year or less.11. The resonator of claim 10 wherein the protective element is a collarof piezoelectric material that surrounds an active interior region ofthe piezoelectric material.
 12. The resonator of claim 10 furthercomprising a Bragg structure formed either above the piezoelectriclayer, below the piezoelectric layer or both above and below thepiezoelectric layer wherein the Bragg structure has a lateral extentthat is at least coextensive with the piezoelectric layer.
 13. Theresonator of claim 10 further comprising a dielectric spacer thatdefines the lateral extent of the active interior region of thepiezoelectric material.
 14. The resonator of claim 10 further comprisinga temperature compensating layer surrounding the piezoelectric layer andelectrodes; wherein the piezoelectric material is supported on thesubstrate by an electrically conductive material structure thatelectrically interconnects one of contact pads or semiconductor deviceson the substrate with the top electrode.
 15. The resonator of claim 10further comprising a cap layer formed over, but not in contact with, thepiezoelectric material surrounded by the temperature compensating layer.16. The resonator of claim 10 further comprising a cap layer formed overthe piezoelectric layer, wherein the piezoelectric layer is separatedfrom the substrate and supported on the substrate by the cap layer. 17.The resonator of claim 10 wherein the cap layer is constructed as aBragg layer.
 18. The resonator of claim 10 further comprising a layer oflow acoustic impedance material formed thereover.
 19. The resonator ofclaim 18 wherein a passivation layer is interposed between the resonatorand the layer of low acoustic impedance material.
 20. The resonator ofclaim 10 further comprising a temperature compensating layer formed overthe piezoelectric layer adjacent to the electrode formed over thepiezoelectric layer.
 21. The resonator of claim 10 wherein theresonator, in operation, has both a parallel resonance and a seriesresonance and wherein at least one of the parallel resonance and theseries resonance changes over a period of time at a rate of about 5 ppmper year or less.